feat(profiling): label-discovery endpoint (POST /profiles/labels)

[FrameAutomata]

What

Adds POST /api/profiles/labels — given {fromDate, toDate, serviceName, type}, returns a map of allowlisted label key → distinct values pulled from profiling_samples.labels, e.g.:

{ "endpoint": ["/a", "/b"] }

Values are deduplicated, sorted ascending, and filtered by project, service, profile type, and time range. Empty values are excluded, and a key with no values is omitted from the response entirely.

The allowlist currently exposes a single key (endpoint); it's a slice so adding keys later is a one-line change.

Why

The dashboard profiles UI needs to discover which label values exist (e.g. which endpoints were profiled) so it can offer them as flame-graph filters. The labels column and label-based filtering already landed on main; this PR adds the read/discovery side.

How

• Repository — DiscoverLabels lives in a build-tag-agnostic file (profile.repository_labels.go); it loops the allowlist and delegates each key to distinctLabelValues, which is implemented per backend: ClickHouse (labels[?], pgch build) and SQLite (json_extract, default build). This mirrors the existing MetricPointRepository.DiscoverTagValues.
• Controller / route — DiscoverLabels handler follows the sibling GetSeries / GetFlameGraph pattern (validates serviceName/type, 400 otherwise).
• The SQLite result model is registered once at startup via models.ExtensionModelRegistrations (alongside profileGroupRow) rather than per request.

Testing

• New profile_labels_test.go (SQLite build): asserts distinct + sorted values, allowlist enforcement (non-allowlisted region excluded), the service/type/time filters, and that a key with only empty values is omitted.
• Both default (SQLite) and -tags pgch (ClickHouse) builds compile; go vet clean.
• Manually verified end-to-end against a local SQLite backend: seeded profiling_samples rows → POST /api/profiles/labels returns {"endpoint":["/a","/b"]}.

🤖 Generated with Claude Code

[FrameAutomata]

Opened against the wrong base — superseded by the upstream PR tracewayapp#238.